1. Field of the Invention
The present invention relates to a brake control system for automotive vehicles, and specifically to a computer-controlled braking system with a skid control unit (an ABS unit) acting to prevent a wheel lock-up condition during vehicle deceleration and to provide maximum effective braking by virtue of a computer-controlled, regulated wheel-brake cylinder pressure based on an arithmetically-calculated deceleration of the vehicle body instead of an input informational data signal from a longitudinal acceleration sensor.
2. Description of the Prior Art
As is well known, in typical skid control systems (ABS systems), rotational wheel speed sensors are employed at each road wheel to sense a rotational wheel speed Vw of each road wheel. In order for the ABS system to arithmetically calculate vehicle speed and then calculate a slippage at each road wheel on the basis of the arithmetically-calculated vehicle speed Vi (often called "pseudo vehicle speed") and wheel speed data of each road wheel, a highest one of the wheel speeds sensed at the respective wheels is often used as a selected value (a reference value) Vfs. The selected value Vfs, corresponding to the highest wheel speed of the wheel speed data signals will be herebelow referred to as a "select-HIGH wheel speed". During skid control, the wheel speed Vw of a certain road wheel (especially subjected to the skid control) tends to vary widely apart from the actual vehicle speed. Also, in order to estimate changes in the pseudo vehicle speed Vi or to compensate for the difference between the actual vehicle speed and the pseudo vehicle speed Vi, an arithmetically-calculated deceleration .DELTA.Vi of the vehicle is often used instead of the use of a signal from a longitudinal acceleration sensor. As a method of arithmetically calculating the vehicle deceleration .DELTA.Vi, the conventional ABS system uses two values or two points, one being a value V0 of the pseudo vehicle speed Vi calculated at the time when the vehicle deceleration is started, and the other being a value Vp of the pseudo vehicle speed Vi calculated at the time when the pseudo vehicle speed Vi changes from an increasing state to a decreasing state at each cycle of skid control. The former value V0 will be hereinafter referred to as a "vehicle-deceleration starting-period vehicle speed", whereas the latter value Vp will be hereinafter referred to as a "spin-up speed" or a "spin-up speed value" or a "return-to-normal speed value". Actually, the vehicle deceleration .DELTA.Vi is calculated as a gradient of the line segment including the above-mentioned two points V0 and Vp, from an expression .DELTA.Vi=(V0-Vp)/T, where T denotes a derivative time (a time interval between the two points V0 and Vp). During execution cycle of the skid control, the arithmetically-calculated vehicle deceleration can be used, rather than using a sensor signal value from a longitudinal acceleration sensor, and thus the system operates in conjunction with a signal indicative of a slippage which is calculated based on both the wheel speed of each road wheel and the pseudo vehicle speed corrected by the arithmetically-calculated vehicle deceleration .DELTA.Vi. In comparison with the use of an expensive longitudinal acceleration sensor, the use of arithmetically-calculated vehicle deceleration .DELTA.Vi is advantageous in reduced production costs of the ABS system. In arithmetically calculating the pseudo vehicle speed Vi based on the select-HIGH wheel speed, there is another drawback. That is, during braking action, there is a tendency for brake resonance noise (rumbling noise) to occur especially at front road wheels. In such a case, a signal indicative of the wheel speed Vw, contains electrical noise. As a result, the select-HIGH wheel speed is set undesirably at an extremely high level, and thus the accuracy of arithmetic-calculations of the pseudo vehicle speed Vi and the vehicle deceleration .DELTA.Vi may be deteriorated. To avoid this, Japanese Patent Provisional Publication No. 7-89428 teaches selection of the previously-noted selected value (the reference value) Vfs from the highest wheel speed in the four wheel speeds sensed each road wheel and the lowest wheel speed (or the second lowest wheel speed data), depending on whether the vehicle speed is within a resonant speed range that there is an increased tendency of occurrence of resonance between sprung and unsprung masses in a vehicle suspension vibrating system and/or in a brake vibrating system. In the system disclosed in the Japanese Patent Provisional Publication No. 7-89428, when the vehicle is running in a non-resonant speed range (i.e., a high-speed range) except the resonant speed range or a skid control system is in operation, the select-HIGH wheel speed is selected as the selected value Vfs by way of a so-called select-HIGH process. In contrast, when the vehicle is running in the resonant speed range (i.e., a low-speed range) and the ABS is in its in-operative state, the lowest wheel speed or the second lowest wheel speed data is selected as the selected value Vfs by way of a so-called select-LOW process. The lowest wheel speed or the second lowest wheel speed will be hereinafter referred to as a "select-LOW wheel speed".